U.S. patent number 6,907,151 [Application Number 10/253,087] was granted by the patent office on 2005-06-14 for optical coupling for a flip chip optoelectronic assembly.
This patent grant is currently assigned to Texas Instruments Incorporated. Invention is credited to Mohammad Yunus.
United States Patent |
6,907,151 |
Yunus |
June 14, 2005 |
Optical coupling for a flip chip optoelectronic assembly
Abstract
A flip chip optoelectronic device assembly includes a hollow,
cylindrical spacer between an optical source in the substrate and
the active surface of the chip, which precludes attenuation of the
signal and allows direct transmission through air. An underfill
material fills the space between chip and substrate, thereby
allowing substrates which are not necessarily matched in thermal
expansion to the chips, and the spacer acts as a dam to prevent
ingress of underfill material into the optical path. The spacer not
only allows use of conventional underfill materials to support the
interconnection joints and thermal mismatch, but also defines a
fixed "z" axis distance between substrate and chip.
Inventors: |
Yunus; Mohammad (Dallas,
TX) |
Assignee: |
Texas Instruments Incorporated
(Dallas, TX)
|
Family
ID: |
31993089 |
Appl.
No.: |
10/253,087 |
Filed: |
September 24, 2002 |
Current U.S.
Class: |
385/14;
385/31 |
Current CPC
Class: |
G02B
6/4214 (20130101); G02B 6/4232 (20130101); G02B
6/43 (20130101) |
Current International
Class: |
G02B
6/42 (20060101); G02B 6/43 (20060101); G02B
006/12 () |
Field of
Search: |
;385/14,31-33,47-49,15 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Font; Frank G.
Assistant Examiner: Mooney; Michael P.
Attorney, Agent or Firm: Tung; Yingsheng Brady, III; Wade
James Telecky, Jr.; Frederick J.
Claims
What is claimed is:
1. An assembly for coupling an optoelectronic circuit to a
substrate including; a) one or more transport devices secured in
said substrate through which optical signals can be carried, and in
turn can be emitted through corresponding apertures on the first
surface of said substrate, b) a plurality of wiring paths and
contact pads defined on the first surface of said substrate, c) one
or more optoelectronic chips connected to said substrate by flip
chip contacts, d) one or more hollow cylindrical spacers coupling
said apertures to active circuits on said chip surface, and e) an
underfill material between the chips and substrate, separated from
the optical signal path by said spacers.
2. An assembly as in claim 1 wherein said optical signal transport
device is an optical fiber.
3. An assembly as in claim 1 wherein said optical signal carrying
device is a wave guide.
4. An assembly as in claim 1 wherein said spacer is a stand-off
member of predefined height.
5. An assembly as in claim 1 wherein said spacers are attached by a
low modulus polymer.
6. An assembly as in claim 1 wherein said flip chip contacts
comprise solder.
7. An assembly as in claim 1 wherein said spacer includes a
reflective film coating the interior walls of the cylinder.
8. An assembly as in claim 1 wherein said underfill material is a
polymer formulated with a filler having no special optical
transmission properties.
9. An assembly as in claim 1 wherein said spacer is a glass
tube.
10. An assembly in claim 1 wherein said spacer is a metal tube.
11. An assembly as in claim 1 wherein said substrate is not
necessarily matched to said chip in coefficient of thermal
expansion.
12. An assembly as in claim 1 wherein said chips include silicon
and gallium arsenide.
13. An assembly as in claim 1 wherein said spacer defines the
distance between said chip and substrate, thereby controlling "z"
axis alignment.
14. An assembly as in claim 1 wherein attenuation of optical
signals between chip and substrate is minimal as a result of direct
air transmission through said hollow spacer.
15. An assembly as in claim 1 wherein said substrate is the base of
a single chip optoelectronic device package.
16. An assembly as in claim 1 wherein said substrate is the base of
a multi-chip module.
17. A method for the assembly of an optoelectronic device optically
coupled to a substrate including the following steps: a) providing
a substrate having interconnection wiring and pads for flip chip
assembly on the first surface, and having an optical transmission
device secured in the substrate which is aligned to an aperture on
the first surface, b) disposing a head of low modulus adhesive to
encircle said aperture, c) providing a hollow cylinder of
predefined height having a bead of low modulus adhesive near the
second end of said cylinder, d) adhering the first end of said
cylinder to the substrate by the adhesive encircling an aperture,
e) aligning and bonding a chip having flip chip solder contacts to
the substrate, and concurrently adhering the spacer cylinder to the
chip surface, and f) dispensing an underfill polymer to fill the
space between chip and substrate, except in the optical path
provided by said cylinder.
Description
FIELD OF THE INVENTION
This invention relates generally to hybrid optoelectronic devices,
and more particularly to optical connections for assembly of such
devices.
BACKGROUND OF THE INVENTION
In communications and computer related fields, a need exists for
combining electrical and optical technologies. In particular,
deregulation of telecommunication operations has resulted in
widespread use of Internet, e-mail, data transmission, cable
networks and wireless telephones. These developments have driven a
strong demand for increased network capacity, speed, and bandwidth,
and to meet this demand, telecommunications services have been
installing optical networks because they anticipate greater
capacity, and more cost effective features than traditional hard
wired networks. Current research anticipates even wider use of high
speed optoelectronics in which photons, rather than electrons will
pass signals from board to board, or chip to chip thereby avoiding
the delays of conventional wiring. Electrical signals from a
processor will modulate a light or laser beam which would shine
through air, a waveguide, or an optical fiber to a photodetector,
which in turn will pass signals on to the electronics.
The resulting demand for optical and optoelectronic components, and
the associated packaging technology to meet the unique needs of
these applications far exceeds current capabilities. Broadband
performance, high density interconnection, and precise alignment of
the optical components present significant challenges to existing
assembly technology. The cost of producing optical and
optoelectronic modules is dominated by the cost of optical
interconnections and packaging the devices, rather than the cost of
the components. Ultimately, the cost of packaging and assembly of
optoelectronic devices will need to be comparable to that for
electronic components, and must rely on much of the technology and
automation from the existing industry.
Currently, the manufacture of optoelectronics modules requires that
an optical fiber be properly aligned to an optoelectronic chip,
namely an integrated circuit. Optical signals received or
transmitted over optical fibers are coupled to an optoelectronic
chip where they are converted to electrical signals. Optical signal
coupling is optimized by precise alignment with minimum
attenuation.
A hybrid optoelectronic package is formed by interfacing an optical
fiber with an optoelectronic device, but preferably direct contact
is not made between the chip surface and fiber end in order to
avoid damage to the chip surface.
A significant aspect of packaging optoelectronic devices or modules
involves aligning an active circuit to an optical fiber or
waveguide, and electrically and mechanically bonding the circuit to
a substrate having interconnection circuitry. One bonding approach
which has been used in electronic components, and which is gaining
favor in optoelectronic devices is flip chip connection by solder
bonding. Flip chip bonding allows for direct connection of the
active surface of a semiconductor device to a substrate. The
contact pads on each chip include solder bumps which are mated to
pads on the substrate, and the solder is reflowed to ensure
electrical and physical contact. Surface tension in the molten
solder causes the opposing contact pads to be aligned with good
precision.
Prior art assembly of optoelectronic devices included one or more
optical fibers positioned on a substrate having wiring
interconnections, and the circuit chip(s) bonded to the substrate
through wire bonds or flip chip interconnection. Some early work on
flip chip assembly of optoelectronic devices provided a silicon
substrate having a groove anisotropic ally etched, the fiber
aligned in the groove, a film deposited to cover and secure the
fiber, and conventional solder mount of a flip chip to the
substrate. Despite the advantage of similar thermal expansion
between the substrate and chip, high cost, misalignment, and some
reliability issues have precluded the success of this process.
It has long been recognized that in order to gain the desired high
frequency response for optoelectronic devices, minimizing the
interconnection distance provides the best approach, and that flip
chip assembly, wherein the active circuit of the chip is aligned
directly atop the end of the fiber, offers the best solution for
these high speed devices.
However, the use of flip chip bonding for optoelectronic assembly
has revealed limitations which were not of concern in electronics,
namely bonding optical and optoelectronic devices involves even
more demanding alignment tolerances than electronics. Alignment in
the plane normal to the substrate ("z" direction) typically has not
been of concern in electronics, but in optoelectronics alignment in
each of the "x", "y", and "z" directions is critical to optical
coupling efficiency.
One form of prior art optoelectronics packaging, illustrated in
FIG. 1a included an aperture 101 formed through a substrate 100,
and an optical fiber 120 secured within the opening. The substrate
with fiber could be aligned in the "x" and "y" directions, but
improper alignment in the "z" direction could allow the fiber to
touch the active surface of an optoelectronic chip 110 and damage
the device. Alternately, in FIG. 1b the end of the optical fiber
121 positioned through the aperture 101 was much further from the
active surface of the chip 111, and could result in misalignment in
the "z" direction, and signal loss.
More recently, attempts have been made to position optical fibers
or waveguides in different types of substrates, and to flip chip
mount the active components onto non-CTE (coefficient of thermal
expansion) matching substrates. This requires use of an underfill
polymer to control stress on the solder joints resulting from the
mismatch. FIG. 2 illustrates the fiber 220 positioned in the
substrate 200 and the optoelectronics chip 210 mounted using flip
chip solder bumps 240. An underfill material 250 having a costly
light transmitting filler surrounds the solder bumps. The assembly
is not unlike that used for flip chip semiconductor devices, except
that attenuation of light is of utmost importance in optoelectronic
device, and therefore, only highly specialized filler for optical
transmission may be formulated into the underfill polymer. While
the specially formulated underfill material attempts to minimize
attenuation of the light beam, it is not as effective as is
required for high speed transmission devices.
Not only is the underfill material very expensive, but a slow
curing process at a low temperature must be used in order to avoid
misalignment of the filler particles and resulting diffraction of
the light. The materials are costly, and the process is both time
consuming and labor intensive, and thus is not amenable to high
volume production.
It would be very beneficial to this fast growing industry if a
reliable, cost effective technique for assembling a flip chip
optoelectronic device were provided.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a method for flip chip
assembly of hybrid optoelectronic devices wherein attenuation of
the optical signal is minimized.
It is an object of the invention to provide a reliable flip chip
assembly process for optoelectronic devices wherein the coefficient
of thermal expansion of the substrate is not necessarily matched to
that of the chip.
It is an object of the invention to control alignment of an optical
fiber or waveguide to an active circuit on a chip in "x", "y" and
"z" directions.
It is an object of the invention to make use of conventional
semiconductor flip chip materials and processes for optoelectronic
assembly.
It is an object of the invention to provide a process for assembly
of flip chip optoelectronic devices which is cost effective, and
further is amenable to high volume production.
It is an object of the invention to provide a flip chip process for
assembly of devices having optoelectronic components either as a
single chip device, or as a multichip module.
It is an object of the invention to eliminate the use of expensive
underfill materials having filler particles which transmit the
light.
These objectives and other are achieved by providing an assembly
including a hollow, cylindrical spacer(s) between the substrate and
chip(s) through which the optical signal from an optical fiber or
other source is transmitted undisturbed, the chip(s) and substrate
are interconnected by solder reflow, and a conventional underfill
polymer surrounds the solder joints and area under the chip, except
the area where the spacer acts as a dam against polymer ingress.
The spacer not only allows use of conventional underfill materials
to support the interconnection joints and thermal mismatch, but
also defines a fixed "z" axis distance between substrate and
chip.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1a illustrates an optical fiber extending through a substrate
and contacting the chip surface. (Prior art)
FIG. 1b illustrates an optical fiber extending to the surface of a
substrate without "z" alignment control. (Prior art)
FIG. 2 is a flip chip optoelectronics device having an underfill
material formulated with specific light transmitting fillers.
(Prior art)
FIG. 3 is a flip chip optoelectronic device of the invention having
a spacer which precludes ingress of underfill into the light
path.
FIG. 4 is a cross section of the spacer showing a reflective
interior surface.
FIG. 5 shows in greater detail attachment of the spacer to chip and
substrate.
FIG. 6 demonstrates attachment of the spacer to the substrate.
FIG. 7 is a single chip embodiment of the invention.
FIG. 8 is a multi-chip embodiment of the invention.
DETAILED DESCRIPTION OF THE DRAWINGS
FIGS. 1a, 1b and 2 have illustrated some prior methods for coupling
a substrate held optical fiber to an optoelectronic chip, and
associated failure mechanisms, such as "z" axis alignment control
in FIG. 1, and attenuation of the signal by an underfill material
in FIG. 2.
FIG. 3 illustrates a device of the current invention including
coupling between an optical fiber 320 held in a substrate 300 and
an optoelectronic chip 310. Electrical connection between the chip
310 contact pads and wiring paths 301 on the substrate are made by
solder bumps 340. An underfill material 350 surrounds each bump and
most of the space between the chip and substrate. A thin walled
cylindrical spacer 360, hollow in the longitudinal direction serves
as a dam to prevent the underfill material from encroaching into
the optical path 390, and allows the light to be transmitted with
little or no attenuation.
The spacer 360 further serves as a stand-off between the chip 310
and substrate 300, thereby accurately controlling the distance and
the "z" axis alignment. Solder connected flip chip devices lacking
a stand-off are subject to height variation resulting from the
weight of the chip, number of contacts, and/or solder reflow
conditions.
An optical fiber 320 positioned in the substrate 300 is aligned to
an active circuit on the optoelectronic device in the "x" and "y"
directions by self aligning solder connections 340, and the "z"
axis by the fixed height of the spacer 360. The spacer allows the
light beam or other optical signal to be unimpeded by underfill 350
or other foreign substance.
The underfill material 350 surrounds the solder connections 340 and
fills the space between chip 310 and substrate 300, except where it
is blocked from the optical path by the spacer 360 which acts as a
dam to prevent ingress of the polymer. An underfill material 350
serves to mitigate stresses on the solder joints arising from
mismatches in thermal expansion between the chip and substrate, and
minimizes movement between the components which could result in
misalignment. The underfill material 350 of the current invention
requires no special optical transmission characteristics, and is a
low cost, conventional filled epoxy typical of those used in
fabrication of semiconductor flip chip devices. Composition of the
underfill is best optimized from properties of the substrate, chip,
and solder.
A cross-sectional view of the spacer 460, in FIG. 4 shows a
reflective coating 480, preferably on the inner surface of the
hollow tubule which redirects stray light back into the optical
path, thereby providing a highly efficient optical interconnection.
Spacers transparent to a specific light beam of the device can be
coated on the outer surface by a reflective film with similar
results.
In FIG. 5, a cross sectional view of the spacer 560 of predefined
height is secured to both the substrate 500 and chip 510 interfaces
by fine beads of low modulus polymer 570/571 respectively. The
preferred low modulus adhesive allows some thermal expansion to
take place within the assembly without putting excessive stress on
the sensitive surfaces to which the spacer is adhered.
In a preferred embodiment, a substrate 600 having wiring paths and
solder bump pads 602 is fabricated with one or more spacers 660
attached by a low modulus adhesive 670 in the area adjacent to an
aperture wherein the signal from the optical fiber 620 or waveguide
emerges, as shown in FIG. 6. Subsequently, the opposite end of the
spacer, having a bead of adhesive 671 on the outer surface is
attached to the chip during flip chip solder reflow.
The current invention for assembly of hybrid optoelectronic devices
is applicable to packaging of a single flip chip 710, as shown in
FIG. 7, wherein the substrate 700 of the optoelectronic device is
the package base, and the package is configured for subsequent
attachment to a circuit board by solder balls 770. The optical
signal to the chip is transmitted through a spacer 760 from an
optical fiber 720 without interference from an underfill
material.
However, the preferred embodiment of the invention includes
assembly a module having multiple solder bump 840 connected flip
chip optoelectronic devices 810 and conventional chips 811 to a
substrate 800 using one or more hollow spacers 860 between the
active circuit and the optical fiber 820 to transmit light without
interference or attenuation, as shown in FIG. 8.
The invention has been discussed most frequently with reference to
an optical fiber as the transport and guide for optical signals,
however, it is amenable to wave guide and other forms of light
transmission.
The hybrid optoelectronic device of the invention anticipates
inclusion of both silicon and gallium arsenide chips, and the
substrate will comprise a ceramic, a polymeric composite, or a
semiconductor material. The flip chip interconnections are
preferably one of many solder compositions, but can also be formed
by conductive polymers. The underfill is a polymeric material,
typical of those used in semiconductor flip chip assembly. The
spacer preferably comprises a hollow glass tube of predefined
length, coated with a reflective film, and attached by a low
modulus polymeric adhesive to the substrate and chip. Inside
diameter of the spacer is preferably in the range of 75 micron to
150 microns, with the wall diameter about 25 to 50 microns. The
glass is an ordinary glass such as that typically found in
laboratory supply catalogue. Alternate materials for spacers are
metals, alloys, or polymers.
One method for assembling an optoelectronic device of this
invention includes providing a substrate having an optical fiber,
wave guide, or other optical transmission device secured in the
substrate, and aligned within an aperture on the first surface of
the substrate.
A thin bead of a "b" staged thermosetting polymer or a
thermoplastic polymer is disposed on the substrate surrounding the
aperture. The adhesive will subsequently be used to adhere the
first end of a hollow spacer to the substrate and allow signal
transmission through the spacer.
A hollow spacer, having a bead of adhesive 671 disposed on the
outer surface near the second end of the spacer is aligned by pick
and place equipment to the adhesive on the substrate, and is
adhered by heat activation, as shown in FIG. 6.
Alignment between the optoelectronic chip and the optical
transmission device in the "x" and "y" directions is achieved by
aligning flip chip solder bumps on the chip with mirroring pads on
the substrate, and heating to reflow the solder. Self aligning
properties of the solder to metallized pad assure "x" and "y"
positioning to within half the pad dimensions.
Simultaneously, during solder reflow, the spacer is adhered to the
chip by thermal activation of the adhesive on the second end of the
spacer, thereby providing "z" axis alignment controlled by the
predetermined height of the spacer. The fixed height of the spacer
controls the distance between chip and substrate, and does not
allow variation in stand-off space as a function of solder reflow
conditions.
An underfill material is dispensed to fill the space between the
chip and substrate, and lend support to the solder joints.
The resulting optoelectronic device is aligned in the "x", "y", and
"z" axes, and the spacer serves as a dam to prevent any
contamination from underfill or other sources from intruding into
the light path while allowing use of conventional packaging
technology. A method supporting high speed transmission of the
optical signal between chip and substrate has been achieved and
other advantageous results attained.
Various changes could be made in the above construction and methods
without departing from the scope and spirit of the invention, but
it is intended that all matter contained in the above descriptions
and drawings shall be interpreted as illustrative and not in a
limiting sense.
* * * * *